Ultrashort Laser Pulses – the Microcosm in Extremly Slow Motion

Wave function for two electrons in doubly excited helium.

How does a quantum system evolve in time and is it possible to visualize or even control its motion? Today, this old dream of physicists from the early days of quantum mechanics has become a real and growing field of research. The time scales of processes elapsing in quantum systems are extremely short: During chemical reactions, the atoms are moving within 10 to 100 femtoseconds (1 fs = 10–15 s), while the electrons which mediate the chemical bond are even faster – here, attoseconds (1 as = 10–18 s) are the characteristic time scale.

The basis for time-resolved experiments are ultrashort intense laser pulses which are used to steer the atomic or molecular dynamics with extremely high precision. Electrons released from an atom by a strong laser field are driven back and forth and revisit its parent ion while probing its structure. The wave nature of the electron leads to interference effects like in a holographic image which can be analysed to resolve the time-dependent interaction with the residual electrons of the atom.

In most cases, a “pump-probe” scenario is applied, where the first “pump” laser pulse prepares the system in the desired way and starts the time evolution which is then probed by the second laser pulse. Molecular motions like vibration and rotation can thus be traced. Observing chemical reactions in real time at femtosecond resolution is a very promising research area. In combination with the REMI technology even the ultrashort time span needed for a so-called isomerization reaction – rearrangement of atoms within a molecule – which is essential also in eye-vision or photosynthesis could be observed.

To observe the motion of electrons, however, even shorter light pulses on the order of attoseconds are required. One possibility therefore is the generation of high harmonics of the wavelength of a femtosecond laser. This way, the requested pulse durations of less than 100 attoseconds at wavelengths of some 10 nm can be reached nowadays. The helium atom represents a prototype for the correlated motion of electrons. Both its electrons can be excited by absorption of ultraviolet attosecond pulses. Another femtosecond laser pulse time-dependently probes the thus generated two-electron wave packet which can be reconstructed by calculations based on known static wave functions. The laser pulses even allow to steer this electronic ‘couple dance’. In the future, a directed manipulation of the electron pairs in molecules may influence chemical reactions and enable hitherto impossible syntheses.

Reaction Microscopes and Laser Systems

Scheme of a reaction microscope.

Reaction microscopes – “the bubble chambers of atomic and molecular physics” – have been developed and are continuously improved at MPIK. Ultra-short intense laser pulses or particle beams induce a breakup of simple molecules. The fragment ions and electrons are caught by means of electric and magnetic fields and recorded by large-area time- and position-sensitive detectors. Their complete momentum vectors, and thus the geometry and dynamics of the molecules before their break-up, can be determined from the reconstructed trajectories of the fragments ("kinematically complete experiments"). The instruments are deployed in-house and are regularly transported for measurements campaigns to external light sources such as free-electron lasers (FELs). For the cryogenic storage ring CSR, a specific reaction microscope has been designed which is presently under construction. It will be a key instrument for the worldwide unique possibilities for the investigation of slow and cold ions in the CSR.

In the Institute’s laser laboratories, phase-controlled laser pulses as short as 5 fs at intensities of up to about 1016 W/cm2 are routinely available for experiments. Even shorter pulses of some attoseconds duration are generated by special nonlinear optical techniques. The resulting coherent high-harmonic radiation in the extreme UV range can then be combined with broadband infrared/visible pulses from the main Ti:Sapphire laser. Isolated as well as double and triple attosecond pulses are produced and used to probe gaseous atomic and molecular samples by interferometric methods. For pump-probe measurements, the time delay between two pulses can be precisely adjusted on attosecond time scales. Combined with spectroscopy or imaging detectors, this allows for direct and time-resolved observation (and control) of nuclear and electronic quantum motions in chemical reactions. The MPIK is one of the three MPG core-partners of the “Max Planck Centre for Attosecond Science”.

In addition, MPIK researchers use the ultraviolet and X-ray pulses of the free-electron lasers in Hamburg (FLASH), Japan (SCSS) and Stanford (LCLS). They also contribute to the future extreme light infrastructure, ELI.

Colliding Atoms and Molecules – Billiard Game with Quantum Balls

Fragmentation of a DNA component by electron impact.

Research on correlated quantum dynamics represents one of the great challenges in contemporary science. Researchers at the MPIK explore quantum dynamics at its very fundamental level, starting from a limited number of few interacting particles in atoms and molecules, and extending to more complex finite quantum systems like clusters or even biomolecules. Bombardment with charged particles (electrons, ions) is a key method for the study of these quantum systems. Novel multi-coincident imaging techniques developed at MPIK provide comprehensive information about few-body quantum dynamics and allow a test of theories for such reactions.

Electron impact plays an important role in the environment, for example in the upper atmosphere and in interstellar space, as well as in technical plasmas and in radiation biology. For the first time, the researchers could observe, in which way the spatial structure and orientation of a molecule influences the direction in which an electron is ejected. During a collision, a molecule may break up in several fragments; this plays a crucial role in biological tissues, since, e. g., the DNA molecule (carrier of the genetic information of a cell) can be altered chemically or even destroyed. MPIK researchers investigate, how the building blocks of DNA break up on bombardment with electrons. The final product depends on which of the electrons is ejected from the molecule. It is expected that the results of these measurements will help to better understand both the emergence of tumors as well as their destruction by radiation therapy.

Quantum Control – Steering Atoms and Nuclei with Lasers

Experiment on nuclear quantum optics with X-ray light.

Spectroscopy – the measurement of the absorprtion and emission of light on its interaction with matter – is one of the most important tools of physics. Line spectra are observed in the case of resonant interaction. Under certain conditions, they interfere with a continuous background and asymmetric line shapes (so-called Fano profiles) emerge. This can be illustrated as the superposition of coupled oscillations. Using ultrashort infrared laser pulses, it is possible to control the temporal evolution and thus the quantum interference – for example as the reversion of absorption to emission.

Quantum optics with X-radiation emerged in the last years as a new field. An important step here is the preparation of robust superpositions of quantum states. Theoretical predictions for this could already be confirmed by an experiment with the atomic nuclei of an iron sample which – embedded in a resonator for X-ray light – could be brought into the desired superposition of states.

Other, still purely theoretical investigations try to solve the question, how the energy of X-ray quanta could be stored in long-lived nuclear states and released again on demand.